1. Field
This invention relates to the mixing of fluids, and is specifically directed to mixing techniques which minimize turbulence. It provides a recursive cascade conduit structure.
2. State of the Art
Turbulence is one of the most important phenomena of fluid motion. Most kinds of fluid flow are turbulent; common examples including process mixing, river flow, fluid jet streams, atmospheric and ocean currents, pump flow, plumes and the wakes of ships. Turbulence is characterized by the development of eddy cascades. The term "cascade" is used in this disclosure to characterize the flow of fluids through a series of regions, progressing from higher to lower energy levels. Within eddy cascades, currents bring about rapid fluctuations within a space and during a time interval, of the physical properties of a fluid. A characteristic of turbulence is the flow of energy from larger to smaller spatial scales. Energy is passed down the eddy cascade to smaller and smaller eddies until the inherent viscosity of the fluid causes dissipation of the energy as heat.
Turbulence is relied upon for a wide range of processes. These processes include heat and mass transfer, fluid distribution and mixing. While useful for such practical applications, turbulence also imposes some limitations and negative characteristics upon the commercial processes in which it exists.
Turbulence is ubiquitous in mixing operations. Molecular diffusion is a very slow process of limited application. "Stretch and fold" techniques are used to mix very high viscosity materials, but have little other practical application. Almost all other forms of mixing involve some form of induced turbulence. Most commonly, mechanical interaction is employed to create a desired level of agitation. Devices for mixing include propeller and stirring devices, aerators, shaking devices, blenders and pumps. Other devices rely upon various configurations of fluid jets, baffles or impinging structures to induce turbulence. Alternatively, the fluids to be mixed may be passed through an apparatus of the type referred to as a "motionless" or "static" mixer. Such devices are static with respect to their structure, but have internal elements arranged to cause inter-fluid turbulence.
Non-turbulent mixing devices are very uncommon, being inconsistent with common experience. U.S. Pat. No. 4,019,721 discloses a mixer characterized as "non-turbulent." The apparatus of that patent operates by passing fluids upwardly into a chamber containing a heavy ball. The disclosure acknowledges that turbulence is probably induced in the fluid on the downstream side of the ball, in addition to other poorly understood non-turbulent mixing effects as the fluid flows around the ball.
Fluid mixing is regarded as a turbulent process, and the efficiency of mixing is regarded as a function of the severity of the turbulence. It is commonly understood that mixing improves as turbulence is heightened. Heightened turbulence is accomplished, for example, by increasing mixer blade speed (increased rpm), shaking fluids more violently, stirring faster, adding turbulence causing baffles and equivalent expedients for adding energy to the fluids.
"Sorption processes" involve the contacting of a fluid stream with a fixed bed of solid particles. In such operations, a solid sorption material is surrounded with a fluid which moves through the voids around and/or within the solid particles. The usual configuration of a sorption process includes columns filled with the solid sorption material. The fluid to be treated is passed either upflow or downflow through the column. A key characteristic of such processes is that entering fluid passes into and through the bed as a moving cross section. Fluid distributors are used to introduce fluid into and collect fluid from the column on an intermittent or continuous basis. U.S. Pat. Nos. 4,999,102, and 5,354,460 disclose recent examples of industrial fluid distributor designs which claim a uniform distribution/collection over a cross sectional area of a column. The goal of these and other similar devices is to distribute and/or collect a two dimensional surface of fluid.
A common approach to rapidly distributing an entire volume of fluid within a bed of sorption material is to induce energetic turbulent mixing. For example, liquid can be added to a bed of solid particles while vigorously stirring or blending the fluid and solid together. While such a turbulent process does accomplish the goal of rapid volume mixing, it also imposes several undesirable consequences. For example, turbulence under these circumstances eliminates the possibility of efficient packed bed operation, because the bed is fluidized. Mechanical attrition of the solid bed particles is inevitably increased. Additionally, if such a process is operated in a continuous manner, there results a ceaseless intermixing of entering untreated material and treated material which would otherwise be suitable for exiting the system. These undesirable features associated with fluidization are avoided by the conventionally preferred method of flowing fluid up or down a packed column under non-turbulent flow conditions.
U.S. Pat. No. 5,307,830 describes a method for reducing turbulence downstream of a partially open or closed valve element. The device comprises a group of identically sized tubes to smooth the turbulence and distribute the resulting fluid to a cross sectional area, rather than to a volume.
It is well known that three dimensional fractal structures of conduit exist in nature. For example, the blood vessels of the heart and the airways of the lung exhibit fractal architecture. The usefulness of this evolved architecture is recognized to include the ability to provide distribution and collection of fluids to the cells of the body (blood vessels) and present a large surface area for gas exchange (lungs). It has not been recognized that such structures can be used as a useful alternative to inter-fluid turbulence. Furthermore, no method has previously been disclosed which describes procedures to design and make practical use of devices of this type.
There remains a need for a device or system which can effect excellent mixing without the disadvantages associated with turbulence.